Another factor is if you are printing a large object your cooling fan is pulling in air off the 'warm' bed, so you are actually blow about 50C air over the cold end. I use an downward angled cooling fan now to help with this issue myself.

I will express my CRAZY ideas at any time! So you have been warned. None of my opinions are Lulzbots and can be wrong at any second.

Is there anything in the environment that may contribute to higher-than-expected temperatures around the extruder? What is the ambient temperature in the room by the printer?

Things to consider:
- Is the printer in direct sunlight?
- under or near a warm-air vent?
- In an enclosure, a closet, or a very small room with little or no air circulation?

Sunlight? No, Warm air? I live in California, we use the heater once every 4 years.
I do have an enclosure, but the problem still happens if I remove it.

Preventing heat creep depends on that little fan blowing cool air on the extruder heat-sink, and if the air that it is blowing is warm enough, the printer may be working as designed but still be unable to keep the temperature of the extruder and filament low enough to prevent heat creep.

Edited to add: Consider this thought experiment -- PLA has a glass transition temperature (the point where it softens) of about 140 degrees F (60 C). The goal is to keep the top of the heat sink below that temperature. If the ambient air temperature is 100 degrees F (in an enclosure, or closet, perhaps), then the differential between the air and the maximum temp is a paltry 40 degrees F. But, if the air temp is held at about room temperature (70 degrees F), that differential jumps up to 70 degrees -- a considerable difference when cooling.

Thanks for the unique explanation. The temperature has definitely been in the 70-80 degree range. We are having a bit of a heat wave. But, I've been having his problem for months now. Since the spring.

Question. Do these types of fans slow down instead of outright dying? I've put a few hundreds hours on this thing. Should I replace the fan?

Another factor is if you are printing a large object your cooling fan is pulling in air off the 'warm' bed, so you are actually blow about 50C air over the cold end. I use an downward angled cooling fan now to help with this issue myself.

Oh that's interesting. Did you print a mod to get this downward angle?

you could try adding a small desk fan or possibly turning down your print temperature slightly. What brands of PLA have you tried? What settings were you using to print?

So, the print that failed most recently with the enclosure removed. I tried that print again with a fan pointed at the printer, it printed almost perfectly. (Tiny bit of stringing, but remember I turned off retraction completely)

One cause of filament grind is having your flow rate too high. I had this exact problem. Reduce your flow rate to the point where on your first layer each nozzle pass of filament just touches the one next to it.

If you overextrude, nozzle pressure builds up and the hobbed bolt is unable to push the filament forward, causing it to take a chunk out of the side of the filament.

I'm experiencing something similar (see viewtopic.php?f=36&t=7696). My prints end up "printing in air" and the filament in the hot end is chewed up. Not 100% sure if my issue is due to the filament getting grinded down and chewed up, but that does seem to be one of the issues.

FWIW, another user (kmanley57) suggested that the issue was that the heat sink wasn't getting cooled enough since it was inside an enclosure (great for ABS, but apparently not so much for PLA). I removed the top and the front panel of my enclosure, and PLA is now printing great again. I haven't printed it yet, but I found a fan for the Taz 4 on thingiverse that cools both the print and the heat fins (https://www.thingiverse.com/thing:431185) and I'm going to give that a try as well.

One cause of filament grind is having your flow rate too high. I had this exact problem. Reduce your flow rate to the point where on your first layer each nozzle pass of filament just touches the one next to it.

If you overextrude, nozzle pressure builds up and the hobbed bolt is unable to push the filament forward, causing it to take a chunk out of the side of the filament.

Jim

I tried this. Default settings had it at 100% flow rate, so I went down to 95%. Still had a failure a few hours in.